Inuyasha was originally a manga, written and drawn by Rumiko Takahashi, one of the world's most talented manga artists. Inuyasha was first published in 1996, in a magazine called Shonen Sunday. The first volume of Inuyasha was printed on May 15, 1997, in Japan. Currently, there are 28 volumes of manga and the series is still ongoing. For those of us in English speaking countries it is harder to read those mangas in Japanese, so Viz bought the rights to Inuyasha in America and is translating the manga. Even though their translation are a watered down version of the original Inuyasha, it's still readable. In Japan, the mangas cost 390 yen. In the U.S (I'm not sure about other countries) you can buy a Japanese Inuyasha for around $6.00. Some good places to buy them are Kinokuniya and Sasuga Bookstore. In English, they cost around $15.95 and you can get them at Ebay and Amazon.com as well as just about any other comic bookstore. Also, if you don't want to pay anything, I've seen the mangas around the Internet (mostly on Streamload request pages though) so if you look hard enough, you could probably get some of the volumes for free. As long as you don't sell 'em it should be alright, however I recommend that you just go out and buy them!
If you would like more information on the priniting dates and other information, please look at the information below.
Japanese Manga
Volume 1: ISBN 4-09-125201-X
First Printing: May 15, 1997
Volume 2: ISBN 4-09-125202-8
First Printing: August 15, 1997
Volume 3: ISBN 4-09-125203-6
First Printing: November 15, 1997
Volume 4: ISBN 4-09-125204-4
First Printing: January 15, 1997
Volume 5: ISBN 4-09-125205-2
First Printing: April 15, 1998
Volume 6: ISBN 4-09-125206-0
First Printing: June 15, 1998
Volume 7: ISBN 4-09-125207-9
First Printing: September 15, 1998
Volume 8: ISBN 4-09-125208-7
First Printing: December 15, 1998
Volume 9: ISBN 4-09-125209-5
First Printing: February 15, 1999
Volume 10: ISBN 4-09-125210-9
First Printing: May 15, 1999
Volume 11: ISBN 4-09-125581-7
First Printing: August 15, 1999
Volume 12: ISBN 4-09-125582-5
First Printing: October 15, 1999
Volume 13: ISBN 4-09-125583-3
First Printing: December 15, 1999
Volume 14: ISBN 4-09-125584-1
First Printing: March 15, 2000
Volume 15: ISBN 4-09-125585-X
First Printing: May 15, 2002
Volume 16: ISBN 4-09-125586-8
First Printing: July 15, 2000
Volume 17: ISBN 4-09-125587-6
First Printing: Sptember 15, 2000
Volume 18: ISBN 4-09-12558-4
First Printing: November 15, 2000
Volume 19: ISBN 4-09-125589-2
First Printing: February 15, 2001
Volume 20: ISBN 4-09-125590-6
First Printing: April 15, 2001
Volume 21: ISBN 4-09-125641-4
First Printing: July 15, 2001
Volume 22: ISBN 4-09-125642-2
First Printing: September 15, 2001
Volume 23: ISBN 4-09-125643-0
First Printing: December 15, 2001
Volume 24: ISBN 4-09-125644-9
First Printing: January 15, 2002
Volume 25: ISBN 4-09-125645-7
First Printing: April 15, 2002
Volume 26: ISBN 4-09-125646-5
First Printing: July 15, 2002
Volume 27: ISBN 4-09-125647-3
First Printing: October 15, 2002
Volume 28: ISBN 4-09-125648-1
First Printing: January 15, 2003
Volume 29: Due in April 2003
English Translations
Volume 1: ISBN 1-56931-262-1
First Printing: March 1998
Volume 2: ISBN 1-56931-298-2
First Printing: July 1998
Volume 3: ISBN 1-56931-340-7
First Printing: December 1998
Volume 4: ISBN 1-46931-368-7
First Printing: May 1999
Volume 5: ISBN 1-56931-433-0
First Printing: October 1999
Volume 6: ISBN 1-56931-491-8
First Printing: May 2000
Volume 7: ISBN 1-56931-539-6
First Printing: September 2002
Volume 8: ISBN 1-56931-553-1
First Printing: January 2001
Volume 9: ISBN 1-56931-643-0
First Printing: August 2001
Volume 10: ISBN 1-56931-703-8
First Printing: December 2002